1. Field of the Invention
The invention relates to a controlling method of calibrating an air-core pointer assembly of an electric meter and the device thereof. In particular, using an external compensating voltage, the deviating angle of the pointer and an imposed voltage for the pointer keep a good linear relation.
2. Description of Related Art
With reference to FIG. 9, an air-core pointer assembly of a conventional electric meter has a rotor 51 with a permanent magnet, a pointer 52 on the rotor 51 and two coils 53, 54. The two coils 53, 54 are wound orthogonally with respect to each other. To distinguish them, the two coils are called the cosine coil 53 and the sine coil 54, respectively. Once a voltage is applied to the two coils 53, 54, they interact with the permanent magnet on the rotor 51 to deflect the pointer 52. On the other hand, when the voltage is removed, the pointer 52 still stays at where it was. The pointer 52 does not return back to its zero point.
With reference to FIGS. 10 and 11, a first conventional driving circuit for the air-core pointer assembly is shown. Both of the coils 53, 54 receive a common central voltage Vdd, which is 5 volts. The two coils 53, 54 also receives corresponding first and second input voltages V1, V2 through a driver 55a, 56a, respectively. The phases in the two input voltages V1, V2 differ by 90 degrees. Taking the central voltage 5 V as the reference point, the peak value has an amplitude of 3 V. The lowest trough also has an amplitude of 3V. Therefore, the voltages V1, V2 across both Cosine and Sine coils swings between 2V and 8V from 0 to 360 degrees. The first and second input voltages V1, V2 can be represented by the following two functions, respectively,V1=V_cosine=5V+3V×cos(θ)V2=V_sine=5V+3V×sin(θ)
With reference to FIGS. 12 and 13, another driving circuit and waveform of the air-core pointer assembly are shown. This example does not require a fixed central voltage. The central voltage Vdd is thus set at 0. Both coils 53, 54 are independently and differentially with an amplitude of 3V. To generate the differential voltages, both ends of each coil 53, 54 are connected with drivers 55a, 55b and 56a, 56b. The two drivers 55a, 55b, 56a, 56b associated with each coil 53, 54 are opposite in phase. The input terminal of each pair of drivers 55a, 55b, 56a, 56b is connected to an input voltage V1, V2. Therefore, the input voltages V1, V2 generate differential voltages opposite in phase on both ends of the coils 53, 54 after passing through the drivers 55a, 55b, 56a, 56b. For example, if the input voltages V1, V2 have an amplitude of 3 V, the voltages received by the two coils 53, 54 swing between +3 V and −3 V from 0 to 360 degrees. They can be represented as:V1=V_cosine=3V×cos(θ)V2=V_sine=3V×sin(θ)
With reference to FIG. 14, if a calibrating magnet 57 is added next to the rotor 51, the calibrating magnet 57 interacts with the permanent magnet on the rotor 51 to force the pointer 52 back to its zero position when no voltages are imposed on the coils 53, 54. Relative to the zero position of the pointer, the calibrating magnet 57 can be disposed at any angle. However, it is generally mounted at a specific position that is easy to be controlled. For example, for single calibrating magnet, the magnet is mounted at the angle of 180 degrees. Even if the calibrating magnet 57 is added, the above-mentioned control circuit can still be used without any change.
With reference to FIG. 15, instead of using a single calibrating magnet 57, it is also possible to use two calibrating magnets 58, 59 disposed at 135 degrees and 225 degrees, respectively, to control the pointer 52. The two calibrating magnets 58, 59 achieve effectively the same effect as the single magnet 57 in FIG. 14. As vectors A and B of the calibrating magnets 58, 59 are opposite in direction, they cancel with each other. However, vectors C and D are along the same direction. They add up. Thus, the magnetic field contributed by the two calibrating magnets 58, 59 is equivalent to the single calibrating magnet 57 disposed at 180 degrees as shown in FIG. 14.
Although the external calibrating magnets 57, 58, 59 can force the pointer 52 return back to its initial position, the linearity in the rotation of the pointer 52 is sacrificed when external voltages are imposed on the coils 53, 54. In other words, the rotation angle of the pointer 52 and the strength of the imposed voltage do not keep a good linearity.
To solve the above-mentioned linearity problem, the U.S. Pat. No. 4,492,920 adds, as illustrated by FIGS. 2 and 5 in that specification, a compensating coil (labeled 41) to the original dual coil structure. When a voltage is applied to the compensating coil, a compensating magnetic field is produced. The polarities of the magnetic fields produced by the compensating magnetic field and the calibrating magnet are opposite to each other. Although this method can improve in the linearity, an additional coil is required. This not only increases the production cost but also the entire volume and weight thereof. Therefore, the approach is not ideal.